1. Field of the Invention
This invention relates to an improved developing device to be used in an electrophotographic copying machine, a facsimile machine, and the like. More particularly, this invention relates to an improved developing device by which a latent image registered on an image forming member is developed in such a manner that a two-component-type developer comprising toners and magnetic carriers mixed up each other is used, and a developing sleeve facing the surface of the image forming member is revolved, and magnetic poles are fixed to the inside of the developing sleeve, and a layer of the developer is so formed over the developing sleeve as to be moved as the sleeve is revolved, and a latent image on the image forming member can be developed by the developer layer under the circumstances of oscillatory electric field.
2. Description of the Prior Art
The two-component type developers comprising toners and magnetic carriers mixed with each other have popularly been used, though the amount of the toners to the carriers should be controlled, because they have the following advantages as compared with one-component type developers comprising magnetic toners without using any magnetic carrier; (1) triboelectrification of toners is readily controllable; (2) toner-cohesion is hardly producible; (3) toner-shift caused by a bias field or the like is effectively controllable; (4) toners are not necessarily contained in magnetic substances; (5) even if the toners need to contain such magnetic substances for preventing fog, it may be too little to contain and color clearness may be obtained when using color toners; (6) when rubbing the surface of an image forming member with a developer layer (i.e., when applying the so-called magnetic-brush method), toner ears are excellently produced by a magnetic brush and are excellent in rubbing property; (7) when the surface of an image forming member is cleaned with a magnetic brush, a satisfactory cleaning effect may be enjoyed.
Two-component developing methods have so far been improved one after another for the purpose of improving developability. As examples thereof, such a method is disclosed in, for example, U.S. Pat. No. 3,890,929, Japanese Patent O.P.I. Publication No. 18656/1980 wherein a development comprising a monocomponent type developer in an oscillatory electric field is applied to that made with a two-component type developer. More particularly, there has been disclosed the developing method wherein a two-component type developer comprised of toners and carriers is introduced between an image forming member (i.e., a member for supporting an electrostatically charged image) and a developing sleeve to develop the image (with or without contact) in an oscillatory electric field. This method is disclosed in, for example, Japanese Patent O.P.I. Publication Nos. 139761/1982, 147652/1982 and 147653/1982.
As compared with the developing methods wherein a monocomponent type developer is used in an oscillatory electric field, the difference of the above-given methods is appreciated from the point of view that no magnetic substance may be mixed in for deriving a transporting force of toner in itself from the magnetic substance. Additionally, the methods are suitable for color developments and the developing characteristics thereof may be more improved according to the variations of an oscillatory electric field. Furthermore, in Japanese Patent O.P.I. Publication No. 67565/1984 that was not laid open to the public when this application was originally filed, there is the disclosure of the developing characteristics obtained by making use of a two-component type developer comprising highly resistive toners and carriers.
Developing devices using such a developer as described above include, for example, those in which a developing sleeve is fixed while a magnet which is provided inside the sleeve and is arranged with a plurality of N, S magnetic poles to the direction of the circumference of the sleeve; those in which both developing sleeve and magnet are revolved together; and those in which such a developing sleeve as mentioned above is revolved while the magnetic poles inside the sleeve are fixed. Among them, those revolving the magnet inside the sleeve, like the first two examples, have the advantage that a developer layer formed on the surface of the developing sleeve is moved wavewise. Therefore, even if the thickness of the developer layer is somewhat uneven, any problems caused thereby may be offset by the wavelike movement of the layer. However, there is the disadvantage that the magnet is revolved at a high speed and, as a great turning effort is needed, vibration is apt to occur. Furthermore, the revolving mechanism has to be complicated, sturdy and large sized. On the other hand, in those having magnetic poles fixed inside, like the latter example, there is not such a problem as described in the revolving magnet example but there is the problem that, if the thickness of a developer layer is uneven, the influence thereof is apt to affect the performance because, when the developer layer is moved, a wavelike undulation is caused by the magnetic poles arranged inside the sleeve and, further, when the magnetic poles are arranged to the positions to the image forming member of the developing sleeve, the same influence is apt to be emphasized in accordance with the variations of a packing rate of the developer. There has, however, been almost no study or research of uniforming or thinning a developer layer in a developing area and, therefore, substantially higher quality image developing characteristics have not been satisfactorily achieved.
Thereafter Japanese Patent O.P.I. Publication No. 91453/1984 discloses a developing method wherein a development is performed by making use of a two-component type developer comprising ferrite carriers and toners in an oscillatory electric field. In this method too, however, the image quality obtained therefrom requires a bit of finishing touch for putting it to practical use.
Further, Japanese Patent O.P.I. Publication No. 121077/1984 discloses a system wherein a development is performed by making use of insulating carriers and toners in an oscillatory electric field. However, there is disclosed only a system in which a magnet roller is rotated inside a developing sleeve. This technique is, therefore, not fully satisfactory from the viewpoint of the stabilization of a developer layer.
This fact is not a serious problem as far as magnetic brush contact type development is concerned. However, this will seriously affect a non-contact type development in image quality, such as image density and the like, obtained therefrom.
Taking the above-mentioned points into consideration, Japanese Patent O.P.I. Publication No. 14263/1985, which was not laid open to the public at the time this application was filed, aims at achieving the uniformalization of a developer layer in such a manner that fixed magnetic poles are provided inside a developing sleeve so as not to form any ears in a developing area. In this case, the weak magnetic force restricts the carriers and conductive carriers and toners used therein to serve as a developer. With this technique, the developer will be broken down and the carriers of the developer will adhere to an image forming member and fogging will occur.
Prior to the present invention, the prior art used to require a finishing touch to put it into practical use and to obtain good image quality. Since the present invention was filed there wave been laid open to the public such techniques which cause fog or the like in the course of a developing process even though closer to the practically useful level. Besides the above, the pressing members are taught in Japanese Patent O.P.I. Publication No. 43027/1979 and 43028/1979 and the like. Additionally, it is particularly desirable to minimize the gap between an image forming member and a developing sleeve or the thickness of a developer layer on the developing sleeve. A substantially higher accuracy has been demanded for thinning such developer layers.
Accordingly, not only is such a pressing member to be provided but it is also required to be arranged in a proper position corresponding to the arrangements of the magnetic poles inside the developing sleeve.